With the advance of recent hydraulic technology, numerous forms of hydrostatic drive systems have been developed for all types of vehicles in either two or four wheel drive systems. Parallel type circuits have some inherent advantages wherein a single pump source supplies two or more wheel motors through a parallel rather than a series flow path. Differential motor drive speed, inherent in all turning vehicles, is of no problem since the individual wheel motors are independently driven through a parallel flow path. The basic disadvantage of a parallel flow system is that if one wheel motor loses traction and begins to spin, it will absorb all of the flow in the system, robbing any torque from the remaining wheels. One system to solve this spin-out problem has been a valve and corresponding circuitry which shifts the flow from a parallel path to a series path locking all wheels together while the machine is in a spin-out condition. This system has its disadvantages in that it is manual in operation and more importantly, that it has a maximum efficiency of 50% tractive effort as compared with a parallel system. Another prior art method is the use of individual flow restrictors or flow limiters in the lines to each of the motors causing an excess back prressure if one of the wheels spins-out. However, such a system is equally inefficient since one wheel can accept one-half of the total system flow before the restrictor means begins to function. Other similar prior art methods are shown in U.S. Pat. No. 3,641,765 and U.S. Pat. No. 3,405,776.
The present invention utilizes a single flow responsive divider valve which divides the flow from a single source to a pair of motors regardless of the flow rate and allows a prearranged flow differential between the motors for cornering without decreasing the system efficiency. The divider valve of the present invention includes a shuttle spool having a pair of fixed orifices in the divided flow path to the respective motors so that the flow rates to those motors can be sensed due to pressure drops thereacross. Whenever the flow rate to one motor sufficiently exceeds the flow rate to the remaining motor, the overall pressure differential on the shuttle spool causes the spool to shift and the variable orifice accepting the excess flow begins to restrict flow to maintain the two flows proportional. The fixed orifices are tapered in one direction so that the reverse flow through the valve is essentially free flowing.
Therefore, the principal object of the present invention is to provide an improved hydrostatic parallel drive system for vehicles which permits differential operation without a substantial efficiency loss.
Another object of the present invention is to provide a hydrostatic spin-out valve which is unaffected by the flow levels of the system.
A further object of the present invention is to provide a flow divider valve which is free flowing in the opposite direction.